Manual Pages  — SDT

Multiple trace points may correspond to a single DTrace probe, allowing programmers to create DTrace probes that correspond to logical system events rather than tying probes to specific code execution paths. For instance, a DTrace probe corresponding to the arrival of an IP packet into the network stack may be defined using two SDT trace points: one for IPv4 packets and one for IPv6 packets.

In addition to defining DTrace probes, the SDT macros allow programmers to define new DTrace providers, making it possible to namespace logically-related probes. An example is FreeBSD's sctp provider, which contains SDT probes for FreeBSD's sctp(4) implementation.

The SDT_PROVIDER_DECLARE() and SDT_PROVIDER_DEFINE() macros are used respectively to declare and define a DTrace provider named prov with the SDT framework. A provider need only be defined once; however, the provider must be declared before defining any SDT probes belonging to that provider.

Similarly, the SDT_PROBE_DECLARE() and SDT_PROBE_DEFINE*() macros are used to declare and define DTrace probes using the SDT framework. Once a probe has been defined, trace points for that probe may be added to kernel code. DTrace probe identifiers consist of a provider, module, function and name, all of which may be specified in the SDT probe definition. Note that probes should not specify a module name: the module name of a probe is used to determine whether or not it should be destroyed when a kernel module is unloaded. See the BUGS section. Note in particular that probes must not be defined across multiple kernel modules.

If ‘-’ character (dash) is wanted in a probe name, then it should be represented as ‘__’ (double underscore) in the probe name parameter passed to various SDT_*() macros, because of technical reasons (a dash is not valid in C identifiers).

The SDT_PROBE_DEFINE*() macros also allow programmers to declare the types of the arguments that are passed to probes. This is optional; if the argument types are omitted (through use of the SDT_PROBE_DEFINE() macro), users wishing to make use of the arguments will have to manually cast them to the correct types in their D scripts. It is strongly recommended that probe definitions include a declaration of their argument types.

The SDT_PROBE_DEFINE*_XLATE() macros are used for probes whose argument types are to be dynamically translated to the types specified by the corresponding xarg arguments. This is mainly useful when porting probe definitions from other operating systems. As seen by dtrace(1), the arguments of a probe defined using these macros will have types which match the xarg types in the probe definition. However, the arguments passed in at the trace point will have types matching the native argument types in the probe definition, and thus the native type is dynamically translated to the translated type. So long as an appropriate translator is defined in /usr/lib/dtrace, scripts making use of the probe need not concern themselves with the underlying type of a given SDT probe argument.

The SDT_PROBE*() macros are used to create SDT trace points. They are meant to be added to executable code and can be used to instrument the code in which they are called.

dtrace -l | sed 1d | awk '{print $2}' | sort -u

dtrace -lv -P sched

SDT_PROVIDER_DECLARE(icmp);

SDT_PROBE_DEFINE1(icmp, , , receive__unreachable,
    "struct icmp *");

SDT_PROBE_DEFINE3(ip, , , receive, "struct ifnet *",
    "struct ip *", "struct ip6_hdr *");

int
ip_input(struct mbuf *m)
{
        struct ip *ip;
        ...
        ip = mtod(m, struct ip *);
        SDT_PROBE3(ip, , , receive, m->m_pkthdr.rcvif, ip, NULL);
        ...
}

int
ip6_input(struct mbuf *m)
{
        struct ip6_hdr *ip6;
        ...
        ip6 = mtod(m, struct ip6_hdr *);
        SDT_PROBE3(ip, , , receive, m->m_pkthdr.rcvif, NULL, ip6);
        ...
}

Consider the ICMP probe discussed above. We note that its second argument is of type struct icmp, which is a type defined in the FreeBSD kernel to represent the ICMP header of an ICMP packet, defined in RFC 792. Linux has a corresponding type, struct icmphdr, for the same purpose, but its field names differ from FreeBSD's struct icmp. Similarly, illumos defines the icmph_t type, again with different field names. Even with the ‘icmp:::pkt-receive’ probes defined in all three operating systems, one would still have to write OS-specific scripts to extract a given field out of the ICMP header argument. Dynamically-translated types solve this problem: one can define an OS-independent c(7) struct to represent an ICMP header, say struct icmp_hdr_dt, and define translators from each of the three OS-specific types to struct icmp_hdr_dt, all in the dtrace(1) library path. Then the FreeBSD probe above can be defined with:

SDT_PROBE_DEFINE1_XLATE(ip, , , receive, "struct icmp *",
    "struct icmp_hdr_dt *");

One of the goals of the original SDT implementation (and by extension, of FreeBSD's port) is that inactive SDT probes should have no performance impact. This is unfortunately not the case; SDT trace points will add a small but non-zero amount of latency to the code in which they are defined. A more sophisticated implementation of the probes will help alleviate this problem.